I don't know why but the code below is working when I have a different query:

$result = mysql_query("SELECT * FROM student WHERE IDNO='".$_GET['id']."'")
   ?>
<?php while ( $row = mysql_fetch_array($result) ) {

    ?>
     <?php list($year,$month,$day)=explode("-", $row['BIRTHDAY']); ?>
    <tr>
    <td width="30" height="35"><font size="2">Month:</td>
    <td width="30"><input name="mm" type="text" id="mm"  onkeypress="return handleEnter(this, event)" value="<?php echo $month;?>">

    <td width="30" height="35"><font size="2">Day:</td>
    <td width="30"><input name="dd" type="text" id="dd" maxlength="25"  onkeypress="return handleEnter(this, event)" value="<?php echo $day;?>">

    <td width="30" height="35"><font size="2">Year:</td>
    <td width="30"><input name="yyyy" type="text" id="yyyy" maxlength="25"  onkeypress="return handleEnter(this, event)" value="<?php echo $year;?>">

And it works when this is my query:

$idnum = mysql_real_escape_string($_POST['idnum']);

mysql_select_db("school", $con);
  $result = mysql_query("SELECT * FROM student WHERE IDNO='$idnum'");

Please help, why do I get the undefined offset error when I use this query:

$result = mysql_query("SELECT * FROM student WHERE IDNO='".$_GET['id']."'")

I assume that the query is the problem because its the only thing that's different between the two.